Process for the dehydrogenation of hydrocarbons



Patented June 18, 1940.

PATENT, .orrlcs PROCESS FOR THE DEHYDROGENATION OF HYDROCARBONS Llewellyn Heard, Hammond, Ind., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Application December 31, 1937,

' Serial No. 182,855

5 Claims. (Cl. -19650) This invention relates to a process for the dehydrogenation of hydrocarbons and mixtures thereof; especially those containing a substantial proportion of parafiins, and more particularly to a process for dehydrogenating light liquid hydrocarbons such as petroleum naphthas in the vapor phase using improved metal chromite catalysts.

It is often desirable to remove hydrogen from hydrocarbons and mixtures thereof without materially craclnng or decomposing them. This is especially true of hydrocarbon mixtures containing a substantial proportion of paraflin hydrocarbons. The normally gaseous parafiins, such as ethane, propane and butane, present in natural and refinery gases are of value principally as fuel, whereas the corresponding olefins are very reactive and may be used as source materials for a great number of valuable products, such as alcohols, halogenated solvents, motor fuels, etc. Naphthas obtained by the straight run distillation of petroleum usually contain considerable proportions of paraffin hydrocarbons, and consequently are not well suited for use as motor'fuels because of their low knock rating, whereas such naphthas after dehydrogenation have a much higher knock rating and consequently are readily marketable as motor fuels.

It is an object of my invention to provide an efiicient process for dehydrogenating hydrocarbons using metal chromite catalysts, and especially metal chromite catalysts which are very active and are easily prepared in reproducible form. Another object is to provide a process for increasing the knock rating of petroleum naphthas containing substantial quantities of paraffin hydrocarbons with a minimum of decomposition to hydrocarbons having a smaller number of carbon atoms. Further objects will be apparent from the following detailed description.

In accordance with my invention, hydrocarbons or hydrocarbon mixtures, particularly those containing substantial quantities of parafiins, are dehydrogenated by passing them in the vapor phase at an elevated temperature over a metal chromite catalyst.

Although my process is applicable to many hydrocarbons and hydrocarbon mixtures, my preferred charging stocks contain a substantial quantity, for example 25% or more, and preferably not less than 40%, of paraffin hydrocarbons. Natural and refinery gases containing such quantities, and preferably at least 60%, of light parafflns having from two to five carbon atoms per molecule may be dehydrogenated by my process to give good yields of the corresponding olefins. Similarly, normally liquid hydrocarbon fractions containing substantial amounts of paraflins with five or more carbon atoms per molecule, such as straight run petroleum naphthas, may be converted bymy processto fractions containing a large proportion of olefin and aromatic hydrocarbons. Straight run heavy naphthas, boiling within the range 200-550 E, which generally have very poor antiknock properties, are particularly suitable charging stocks. Such charging stocks are preferably sweetened before treating, by my process, although hydrocarbon fractions which are sour to doctor solution may be used. When such sour fractions are treated according to my invention, the catalysts used have been found to have a substantial sweetening and desulfurizing as well as a dehydrogenating action.

My process is carried out at elevated temperatures, e. g. at temperatures inthe range 650 F. to 1200 F., depending upon the charging stock, feed rate, etc, In general temperatures in the upper portion of this range, e. g. 950 F. to 1200 F., are most suitable for the treatment of normally gaseous charging stock, while temperatures of 650 F. to 1050 F., and preferably 850 to 1000 F., are most suitable for liquid hydrocarbons such as petroleum naphthas. The pressureusedispreferably substantially atmospheric, although a pressure somewhat above atmospheric may be employed in some cases, and subatmospheric pressures are advantageous in some instances because they favor the dehydrogenation reaction. When an elevated pressure is used, the temperature employed should be somewhat higher than optimum for a particular catalyst and charging stock at atmospheric pressure, and the converse is true of subatmospheric pressures. The feed rate is preferably such that the hydrocarbon vapors or gases pass over the catalyst with a contact time of 1 to 5 seconds. For the same catalyst and charging stock, a somewhat greater feed rate may generally be used if the temperature is raised slightly.

The catalyst used in my process are metal chromites having as metallic constituents such metals as copper, silver, gold, cadmium, magnesium, beryllium,calcium,strontium,zinc, cobalt, or nickel, or a mixture of such metals prepared by the thermal decomposition of a complex metal chromate in crystal form.

Metal chromites have, 'of course, been used as catalysts for a variety of chemical reactions, such as the synthesis of methanol from mixtures of carbon monoxide and hydrogen. T1 3 metal iii chromite catalysts heretofore known in the art have been prepared by a number of methods. One such method consists of adding just enough ammonium hydroxide to an aqueous solution containing a metal salt and chromic acid anhydride to form a precipitate of the metal ammonium chromate, which is then filtered off, washed,dried,and ignited to form the corresponding metal chromite. This ignition is generally carried out until the precipitate starts to decompose, and thereafter the decomposition is allowed to continue spontaneously, leaving a glowing residue.

Chromite catalysts have also been prepared by co-precipitation of the hydroxides or carbonates of chromium and other metals, by treating a metallic oxide with chromium trioxide in the wet way, or by fusing a mixture of alkali bichromates and metal oxides, and igniting the resulting masses at h gh temperatures, for example, up to 1000 0., often in the presence of reducing agents.

The metal chromite catalysts obtained by these known methods are quite variable in composition and particularly in physical structure, so that two consecutive batches of catalysts made by any of these methods do not have the same activity, probably because of variations in surface area.

Improved catalysts of the metal chromite type, which are reproducible and have very large surface areas and great catalytic activity, may be prepared by the thermal decomposition of com- "plex metal chromates in the form of well-defined crystals, and it is these catalysts that I use in my process. By complex metal chromates I mean metal chromates having loosely combined therewith other compounds such as water or ammonia. I have found three types of complex metal chromates which give reproducible and catalytically active metal chromites upon thermal decomposition. The first type is a hydrated metal ammonium chromate, the second an ammonio metal ammonium chromate, and the third an ammonio metal chromate.

The preparation of reproducible metal chromite catalysts from hydrated metal ammonium chromates and from ammonio metal ammonium chromates is claimed in my copending applications, Serial Nos. 180,302 and 180,301, respectively, filed December 17, 1937.

My preferred catalysts are prepared from the first type of complex metal chromates mentioned above, 1. e., hydrated metal ammonium chromates, and preferably metal ammonium chromate hexahydrates in crystal form. These hexahydrates have the general formula where M represents a bivalent metal such as magnesium, calcium, cobalt, or nickel, or a plurality of such metals.

One method of preparing these hydrated metal ammonium chromate crystals consists in mixing aqueous solutions of a suitable salt, such as the chloride or nitrate, of one or more of the above metals and ammonium chromate, and allowing the solution to stand until the crystals have formed. These crystals grow quite rapidly to a. considerable size, which is quite desirable, as will be pointed out below. The metal salt and ammonium chromate solutions are preferably quite concentrated and are mixed in approximately stoichiometrie proportions, although an excess of the ammonium chromate ma be used. Finally the crystals are filtered off, washed and dried.

The formation of the metal chromite catalysts for use in my process from these hydrated metal ammonium chromate crystals is accomplished by heating them until water and ammonia are no longer evolved. The temperature used is usually in the neighborhood of red heat, although temperatures as low as 400 F. may be used. The cobalt and nickel chromites thus prepared are finely divided gray powders, but the magnesium and calcium chromites are black rigid solids in which the crystal structure of the hydrated metal ammonium chromates is retained. It is thus seen that when the magnesium and calcium ammonium chromate hexahydrates are allowed to grow to a considerable size, e. g., 4-8 mesh, the magnesium and calcium chromites produced therefrom are of a similar size and are suitable for direct use as catalysts in my process. These catalysts do not break down or pulverize during use. My preferred catalyst is magnesium chromite prepared by the thermal decomposition of magnesium ammonium chromate hexahydrate crystals, because the crystals of the corresponding calcium compound form somewhat less readily. My preferred magnesium chromite catalyst has the additional advantage that it may be prepared in less than two hours, which is a small fraction of the time necessary for the preparation 'of chromite catalysts by methods taught by the art.

In preparing metal chromite catalysts from the second type of complex metal chromates mentioned above, i. e., ammonio metal ammonium chromates, these chromates in crystal form are heated until they are substantially completely decomposed. I preferably use ammonio metal ammonium chromates in which the metallic constituent is copper, silver, gold, cadmium, magnesiumvberyllium, calcium, strontium, zinc, cobalt or nickel, or a plurality of such metals.

The crystalline ammonio metal ammonium chromate from which my catalyst is prepared may be obtained in a number of ways. In my preferred method of obtaining these crystals, however, aqueous solutions of a suitable salt of one or more of the above-mentioned metals and ammonium chromate or chromic acid anhydride are mixed, preferably in approximately stoichiometric proportions although an excess of the chromium compound may be used, concentrated ammonium hydroxide is added until the resulting precipitate of metal ammonium chromate is redissolved and a clear solution formed, and this solution is allowed to stand until crystals of the ammonio metallic ammonium chromate form. The crystals are then filtered off, washed with dilute ammonium hydroxide solution, and dried. The metal salt used is preferably the nitrate, chloride or sulfate, although acetates or other water-soluble salts may be used.

The formation of the metal chromite catalysts for use in my process from these ammonio metal ammonium chromate crystals is accomplished by heating them until they are substantially completely decomposed. These crystals have the property of decomposing spontaneously with the production of incandescence or glowing if they are heated above a temperature ranging from about 450 F. to about 550 F., depending upon the particular metal involved. The extreme heat produced during "g1owing causes a sintering of the fine particles a metal chromite produced by the decomposition and reduces their surface area and consequently their catalytic activity.

The decomposition step therefore is preferably carried out with considerable care in order to avoid the "glow phenomenon. The crystals are subjected to controlled heating, preferably in a thin layer of the order of one-sixteenth of an inch, until they begin to decompose and give off water, ammonia and oxides of nitrogen, and the source of heat is then removed or. reduced so that the temperature is maintained somewhat below the temperature of glowing or spontaneous decomposition, and preferably not substantially above 425 F., until the decomposition is substantially complete. It is advantageous to stir the material during and immediately after the heating step in order to avoid localv overheating. The decomposition may also be carried out at a subatmospheric pressure since the decomposition temperature is thereby lowered and danger of glowing of the material is reduced.

Use of a stream of inert gas, e. g., flue gas or other gas containinglittle oxygen, is also helpful inavoiding glowing during decomposition. The decomposition products are voluminous, finely-divided metal chromites in powder form, which may be washed with hot hydrochloric acid to remove traces of unreacted metal oxide and dried prior to use as a catalyst.

In preparing metal chromite catalysts from the third type of complex metal chromates mentioned above, i. e., ammonio metal chromates, these chromates in crystal form are decomposed at elevated temperatures. Preferably I use crystalline ammonio metal chromates prepared from complex compounds of metal dichromates with organic n1-'- trogen bases such as pyridine, aniline, quinoline, aliphatic amines, etc.

One method of preparing these ammonio metal chromate crystals consisting in mixing concentratel aqueous solutions containing substantially equimolecular quantities of a suitable salt. of a bivalent metal, such as nickelous sulfate, and potassium or sodium dichromate, and adding an excess of an organic base. A crystalline compound separates out, which, if nickelous sulfate, potassium dichromate and pyridine are used, is orange in color and is believed to have the formula NiCr2Ov-4C5H5N. Sufficient concentrated ammonium hydroxide is then added to redissolve the crystals and produce a clear solution. The pyridine Or other nitrogen base separates as an upper layer and is removed by decantation. The clear solution is allowed to stand for a-few hours, and

a complex ammonio metal chromate crystallizes out in' well-defined crystals, which are filtered oil and dried.

The formation of the metal chromite catalysts for use in my process rfom these ammonio metal chromate crystals is accomplished by heating them until they are substantially completely decomposed at an elevated temperature, e. g., at red heat. In the case of the nickel derivative,

the crystals are dark green in color and decomthey may be mixed with other catalysts or inert materials.

The products obtained by dehydrogenating hydrocarbons and hydrocarbon mixtures according to my invention contain increased proportions of unsaturated compounds. In dehydrogenating hydrocarbongases containing substantial quantities of paraflins, the olefin content is greatly increased and in some cases diolefins and/or unsaturated cyclic compounds are formed. The product of the dehydrogenation of petroleum naphthas according to my invention contains both olefins and aromatics, and although some of these aromatics may be formed by dehydrogenation of naphthenes or cycloparaflins present in the charging stock, a considerable proportion of them are believed to be formed by the further dehydrogenation and rearrangement of the olefins produced by the dehydrogenation of the parafiins in the charging stock.

In practicing my invention the hydrocarbons to be dehydrogenated may be passed under the re-' action conditions hereinabove described or at a somewhat lower temperature over a solid adsorbent material such as bleaching earth, fullers earth, activated clay or bauxite, silica gel, etc. prior to contacting with my chromite catalyst. I prefer to use an adsorbent material of the clay type such as Attapulgus clay at suitable temperatures ranging from'about 500 F. to about 850 F. By using this preliminary treatment together with my chromite catalyst I am able to carry out the dehydrogenation using a higher feed rate for a longer period of time than is possible without such treatment.

The following table (Table I) gives the results hexahydrate crystals. In both of these runs the hydrocarbons were passed down through a heated catalyst chamber at atmospheric pressure. In the first run the chamber contained magnesium chromite alone, and in the second it contained a bed of Attapulgus clay above the bed of magnesium chromite. The charging stock used was a sweetened straight run naphtha fraction containing predominantly hydrocarbons having eight carbon atoms per molecule, of which about 50% were paraflins, 30% naphthenes, and 20% aromatics, and having a refractive index of 1.4188. The dehydrogenation activity of the catalyst was determined in these runs by the rate of evolution and the hydrogen content of the permanent gas produced, and the refractive index of the liquid product, which has been found to increase upon dehydrogenation.

, Table I Magne- Ma gneslum figi cbromite+clay Volume .of catalyst cubic centimeters" 50 50 Volume of clay do. 50 Temperature .l F 890 890 Feed rate cc./l1r 20 45 Length of run ..hours 5 7, 5 Refractive index of product 1. 4208 1. 4206 H content of gas cvolved percent 85 Rate of gas evolution:

" After 6 hr cc.ll.u' l, 800 3, 400 After 7.5 hr do. 2, 700

, From the above it is apparent that the magnesium chromite alone was an excellent dehydrogenation catalyst for a period of five hours, and

that the use of the preliminary treatment with clay allowed an increased feed rate for the same degree of dehydrogenation and simultaneously used in the preceding runs, which had an octane number of 38, was passed over 50 cc. of this catalyst at 860-880 F. at atmospheric pressure using a liquid feed rate of 30 cc. per hour. Permanent gases containing 85% hydrogen were evolved at the rate of 4000 cc./hr. and the product had an actane number of 53.2, an increase of 15.2 octane numbers.

A similar run was made using a magnesium nickel chromite prepared from mixed magnesium nickel ammonium chromate hexahydrate crystals. This catalyst retained its dehydrogenating activity for 12 hours, giving ofi gases which were about 80% hydrogen, and producing a product having an octane number of 52.5, an increase of 14.5 octane numbers.

The following table (Table II) gives further examples of the dehydrogenating activity of my chromite catalysts. Another portion of the same naphtha was used in these runs, which were also made at atmospheric pressure. Catalyst I was nickel chromite prepared by heating (NH4)2Ni(C1O4)z-6H2O crystals. Catalyst II was nickel chromite prepared by heating (NHi) 2Ni(CrO4) 2' 2NH3 crystals. Catalyst III was nickel chromite prepared by heating the ammonio nickel chromate crystals produced by adding ammonium hydroxide solution to NiCI207'4C5H5N.

Table II Catalyst I II III Volume of catalyst". .cubic centimeters. 50 50 50 Temperature F. 890 890 890 Liquid feed rate cc./hr. 30 30 30 Gas evolution do. 9,000 8, 000 10, 000 Hydrogen content of gas "percent. 75 60-70 75-80 These examples show clearly the selective action of these chromite catalysts in removing hydrogen from hydrocarbon mixtures containing substantial quantities of parafiins and particularly their suitability for dehydrogenating petroleum naphthas.

The catalysts used in my process may be regenerated after they have become relatively inactive by various methods. One suitable method is to pass air, either alone or diluted with an inert gas, over the catalyst in order to burn off the carbonaceous matter present. The temperature is pref erably controlled, either by control of oxygen concentration or rate of gas flow, so that the temperature during regeneration does not substantially exceed 1000 F.

The apparatus in which my invention is carried out is preferably of the coil and drum type, in which the charging stock is vaporized and heated to reaction temperature by passing it through a pipe coil in a furnace and. the vapors are passed through a reaction chamber containing my dehydrogenation catalyst to apparatus for fractionation or other treatment. It is apparent that other types of equipment may be used and that one skilled in the art can readily design and construct such equipment. When the preliminary treatment with an adsorbent is used, a layer of the adsorbent may be placed above the catalyst bed and the vapors passed down through both layers, or the adsorbent may be placed in a separate chamber and the vapors therefrom contacted with the catalyst, preferably after further increase in temperature.

While I have described my invention in connection with certain specific embodiments thereof, and have stated certain theories relating thereto, I do not desire to be limited thereto but only by the scope of the following claims which should be construed as broadly as the prior art will permit.

I claim:

1. The process of dehydrogenating hydrocarbons which comprises contacting said hydrocarbons in the vapor phase at an elevated temperature in the range 650 F. to 1200 F. with a metal chromite catalyst prepared by the thermal decomposition of a crystalline metal ammonium chromate hexahydrate wherein' the metallic constituent is one or more metals selected from the group consisting of magnesium, calcium, cobalt and nickel.

2. The process of dehydrogenating hydrocarbons which comprises contacting said hydrocarbons in the vapor phase at an elevated temperature in the range 650 F. to 1200 F. with a magnesium chromite catalyst prepared by the thermal decomposition of magnesium ammonium chromate hexahydrate in crystal form.

3. The .process of dehydrogenating liquid hydrocarbons which comprises contacting said hydrocarbons in the vapor phase at an elevated temperature in the range 650 F. to 1050 F. with a metal chromite catalyst prepared by the thermal decomposition of a crystalline metal ammonium chromate hexahydrate wherein the metallic constituent is one or more metals selected from the group consisting of magnesium, calcium, cobalt and. nickel.-

4. The process of dehydrogenating a petroleum naphtha which comprises contacting said naphtha in the vapor phase at an elevated temperature in the range 850 F. to 1000 F. with a metal chromite catalyst prepared by the thermal decomposition of a crystalline metal ammonium chromate hexahydrate wherein the metallic constituent is one or more metals selected from the group consisting of magnesium, calcium, cobalt and nickel.

5. The process of dehydrogenating a petroleum naphtha which comprises contacting said naphtha in the vapor phase at an elevated temperature in the range 850 F. to 1000 F. with a magnesium chromite catalyst prepared by the thermal decomposition of magnesium ammonium chromate hexahydrate in crystal form.

LLEWELLYN HEARD. 

